The Mars Volta wrapped day two at Outside Lands' Twin Peaks stage with a thundering and delirious 90-minute set that drew Metallica's Lars Ulrich and Robert Trujillo and their U.K. tourmates Mastodon to the side of the stage to gawk.

The band, led by Omar Rodriguez-Lopez and vocalist Cedric Bixler-Zavala, maxed out allowable sound levels with their theatrical, histrionic brand of meandering, mystical loud-soft prog-metal. Set against a patterned backdrop of skulls and fishbones surrounding an ominous, Indo-Mayan visage that changed moods with the stage lighting, Bixler-Zavala wailed "If you could see where I've been! during "Ilyena." Fans got a lovely taste of "The Widow," but the real treat came when the El Paso, Texas band dedicated "Luciforms" off of new Octahedron to metal heavyweights Mastodon.

Mastodon themselves turned in the heaviest set of the three-day Outside Lands proceedings Saturday at 3:55 p.m. Even though twin mosh pits erupted during the set, "It was crap," joked lead singer Troy Sanders afterward. "We were playing in the day. You can't rock in the day."

Sanders said Mastodon's September break after months of touring would be filled with kicking back and catching up on movies like Halloween 2 and relaxing after the release of their latest LP Crack The Skye. Mastodon hit the road again with High on Fire and Cartoon Network's virtual band DethKlok in October, and Sanders said many shows are sold out and new dates are being added.

The cartoon that birthed DethKlok, Metalocalypse, pokes fun at narcissistic musicians and the tropes of metal, but Sanders says he's totally cool with it. "I'm happy to play with them. I've got [portly Dethklok drummer] Gene [Hoglan]'s number right here in my phone. I call him and I go, 'How do you play with those ankleweights, man'!" None of the bands will "headline," Sanders says, but will take turns playing hour-long sets.

TV On The Radio's set also marked the end of a year of touring behind Dear Science. "This is going to be our last show for a while. We're glad to share it with you," said Tunde Adebimpe. "Staring at the Sun" and "Young Liars" showcased Kyp Malone in fine form.

Also on the Twin Peaks stage, Tom Morello and Boots Riley's Street Sweeper Social Club had a few things to say. Clad in matching, militant black and orange coats, the electric duo bounced on material from their 2009, debut self-titled release, shouting "100 Little Curses" at thousands of fans who were happy to shout back. "Somewhere In The World It's Midnight" gave Morello plenty of time to demonstrate his legendary guitar squonks and chops. The band covered M.I.A.'s "Paper Planes" less effectively than their pummeling take on LL Cool J's "Mama Said Knock You Out". "It's our first big festival up here," offered Morello. "Thanks for making us feel welcome."